1. Technical Field
The present invention relates to a light-emitting device, and particularly, to an electroluminescence device and to a display using the light-emitting device.
2. Background Art
Recently, attention is focused on an electroluminescence device (hereinafter referred to as an “EL device”) as a light-weight and thin plane-emitting element. The EL device is largely classified into an organic EL device which emits light by applying a d.c. voltage to a fluorescent body made of an organic material to recombine electrons with holes and an inorganic EL device which emits light by applying an a.c. voltage to a fluorescent body made of an inorganic material to collide electrons accelerated in an electric field as high as 106 V/cm with an emission center of the inorganic fluorescent body, thereby exciting the electrons to allow the inorganic fluorescent body to emit light in the course of relaxation of the excitation.
Moreover, this inorganic EL device includes a dispersion type EL device in which inorganic fluorescent body particles are dispersed in a binder made of a high-molecular organic material to form a phosphor layer and a thin film type EL device in which an insulation layer is formed on one or both sides of a thin-film phosphor layer about 1 μm in thickness. Among these EL devices, the dispersion type EL device attracts considerable attention from the reason that it is advantaged in low production costs because of small power consumption and easy production. In the meantime, the dispersion type EL devices conventionally used each have a laminate structure and constituted by laminating a substrate, a first electrode, a phosphor layer, an insulation layer and a second electrode in this order from the substrate side. The phosphor layer has a structure in which inorganic fluorescent body particles such as ZnS:Mn are dispersed in an organic binder, and the insulation layer has a structure in which a strong insulation material such as BaTiO3 is dispersed in an organic binder. An a.c. power source is disposed between the first and second electrodes and a voltage is applied across the first and second electrodes from the a.c. power source to allow the dispersion type EL device to emit light.
In the structure of the dispersion type EL device, the phosphor layer is a layer determining the luminosity and efficiency of the dispersion type EL device and particles having a diameter of 15 to 35 μm are used as the inorganic fluorescent body particles of the phosphor layer, as shown in International Patent Publication No. WO03/020848. Also, the color of light emitted from the phosphor layer of this dispersion type EL device is determined by the inorganic fluorescent body particles used in this phosphor layer. When, for example, ZnS:Mn is used as the inorganic fluorescent body particles, the phosphor layer emits orange light. When, for example, ZnS:Cu is used as the inorganic fluorescent body particles, the phosphor layer emits bluish green light. In this manner, an emission color is determined by the inorganic fluorescent body particles to be used. Therefore, in the case of intending to obtain an emission color, for example, a white color, other than the color of the above emitting light, the color of emitting light is converted into another color by, for example, mixing an organic dye in an organic binder to thereby obtain the intended color, as shown in Japanese Patent Laid-open Publication No. H7-216351.
However, the light-emitting material used for the dispersion type EL device has problems concerning low luminosity and short life.
There is an idea of increasing the voltage to be applied to the phosphor layer as the method for increasing luminosity. In this case, such a problem arises that the half-value period of the light output of the light-emitting material is reduced in inverse proportion to the applied voltage. On the other hand, there is an idea of dropping the voltage to be applied to the phosphor layer as the method for increasing the half-value period, that is, for lengthening the life. However, this brings out the problem as to deteriorated luminosity. As mentioned above, there is a contradictory relation between the luminosity and the half-value period: when it is intended to improve one of these characteristics by increasing or decreasing the voltage applied to the phosphor layer, the other is deteriorated. It is therefore necessary to select either one of the luminosity and life (half-value period of the light output). In this specification, the term “half-value period” means the time required until the light output of the light-emitting material is reduced to the half of the original luminosity.
In light of this, there is a proposal on a method in which an EL device is made to emit light at a low voltage, as shown in Japanese Patent No. 3741157. This EL device 50 is based on a method in which, as shown in FIG. 13, a phosphor layer 53 produced by dispersing light-emitting particles 61 of CdSe microcrystals in a medium of indium tin oxide 63 which is a transparent conductor is interposed between electrodes 52 and 54 and a voltage is applied to the phosphor layer 53 to emit light. Because this EL device 50 is a current injection type light-emitting device, it can be driven at a low voltage.